Silica is useful in reinforcing elastomers such as rubbers and in improving the resistance of rubbers to abrasion. The high structured precipitated silica having fine particle size is particularly used as antiblocking agent for polypropylene and polyethylene films having very high transparency. They are also used as a carrier for dentally active substances, which are stored at the site of action and then release the active substance in small doses over a relatively long period of time (deposition effect, controlled release). Thus, the silica acts as active substance stores which contain the active substance in absorbed, adsorbed or chemisorbed form. Silica is also used in catalysis, inks, papers and in the food industry.
US Patent (Pub.) No. 2006/0027142 A1 (Feb. 9, 2006) assigned to Huang Yung Hui describes new method of producing narrower particle size distribution precipitated silica and/or silicate materials. Such a method permits a significant reduction in manufacturing costs through the utilization of more efficient drying/evaporation components, in essence, in one potentially preferred embodiment, production of such silica and/or silicate materials followed directly by a hydraulic chamber press filtering step combined with vacuum dewatering subsequently leading to the needed resultant particle comminution. The resultant precipitated silicas and/or silicates produced thereby exhibit greater density prior to comminution and thus greater propensity for more uniform milling than previously utilized and prevalent spray/flash drying apparatuses. Also encompassed within this invention are the resultant precipitated silica and/or silicate particles produced, including those that surprisingly exhibit extremely high transmittance properties, thereby potentially providing excellent narrow particle distribution silica materials for clear (transparent) end-uses. This process involves post-synthesis treatment to get the narrow particle distribution using a hydraulic chamber press-filtering step combined with vacuum dewatering. This will add to the cost of production.
US Patent (Pub.) No. US 2005/0228106 A1, (Oct. 13, 2005) assigned to Schaefer et al. describes a method for the preparation of readily dispersible precipitated silica with primary particles of about 5 nm radius, having BET specific surface in the range of 280-350 m2/g and bulk density in the range of 0.4 to 0.6 g/cc. The precipitated silica in this invention is obtained by neutralization of sodium silicate solution in different steps under reduced ionic strength and at elevated temperature in the range of 60 to 100° C. The neutralizing agents used are sulfuric acid and silicic acid. This process involves surface modification of primary particles and aggregates by organo-silanes in order to prevent agglomeration of the aggregates to obtain hydrophobic surface with size of the particles in the colloidal range. The primary particles are measured by small angle X-ray scattering (SAXS) technique; however no mention has been made for the size of secondary (aggregates) or tertiary (agglomerates) particles. In this invention the use of silicic acid in one step of the process makes process complicated, as silicic acid is prepared by passing the silicate solution through a column of cation exchange resin and this solution is unstable and cannot be stored for the longer time. Silicic acid thus obtained should be used immediately and can be stored at lower temperature for few hours only. Another drawback is that the reaction time is longer and it takes about 150 to 200 minutes for complete neutralization.
U.S. Pat. No. 6,902,715 (Jun. 7, 2005) assigned to Maus, et al. describes an invention which provides silica particles, which include the following physical properties: BET surface area: 100-700 m2/g; DBP absorption: 100-500 g/100 g; tamped density: 100-250 g/l; ALPINE sieve residue>63μ: <5%; and particle sizes (cumulative volume distribution): d95<40 μm; d50<20 μm; and d5<10 μm. The present invention also provides a process for producing silica having a narrow particle size distribution, which includes drying a silica suspension in a pulse combustion dryer to produce silica particles having particle size distribution (cumulative volume distribution): d5<10 μm; d50<20 μm; and d95<40 μm. According to this invention fine size particles are achieved by post-synthesis treatment, which required special type of pulse combustion dryer. This may add to the cost of production.
U.S. Pat. No. 7,037,476 (May 2006) Raksh Vir Jasra et al. describes a new process for the preparation of free flowing hydrated amorphous silica from Kimberlite tailing—a waste in diamond mining. The process comprises treating of Kimberlite with acid, further reacting it with alkali solution to obtain soluble metal silicate solution, which is subsequently neutralized with mineral acid to polymerize silica as insoluble precipitates. The product is useful in rubber, paints as abrasive etc. The drawback, of the process is that the treatment of kimberlite with acid, filtering & washing to remove adhering soluble salt and the dissolution of silica in alkali at high temperature, are time consuming which increases the over all time of one batch. More numbers of unit operations, adversely affects the economics of the process.
US Patent Application No. 20030118500 (2003) assigned to Chevallier, et al. discloses a process for the preparation of precipitated silica, comprising the reaction of a silicate with an acidifying agent, whereby a suspension of precipitated silica is obtained, followed by separation and drying of this suspension, characterized in that the precipitation is carried out in the following way: (i) an initial stock solution is formed containing at least some of the total amount of the silicate used in the reaction and at least one electrolyte, the concentration of silicate (expressed as SiO2) in the said initial stock solution being between 50 and 60 g/l, (ii) the acidifying agent is added to the said stock solution until a pH of between 7 and 8.5 for the reaction medium is obtained, (iii) the acidifying agent is added to the reaction medium along with, where appropriate, simultaneously, the remaining amount of the silicate, an additional amount of acidifying agent is added to the reaction medium, preferably until a pH of between 4 and 6 is obtained in the reaction medium. The separation comprises a filtration and washing operation using a filter equipped with a means of compacting, a suspension having solids content of less than 17% by weight is dried by spraying. The drawback of the process is that both the solutions are to be added simultaneously and their rates are to be controlled very critically. Slight variation in rate of addition may affect the pH of the reaction mass and subsequently the quality of silica. Spray drying is required for obtaining silica of desired particle size. This requires high energy input.
U.S. Pat. No. 6,468,493 (2002) assigned to Chevallier, et al. describes the process for the preparation of precipitated silica, which comprises of the following steps i) an initial stock solution is prepared containing some of the total amount of silicate used in the reaction and at least one electrolyte. The silicate concentration in terms of SiO2 in the said initial stock solution is in the range of 40 to 330 g/l and electrolyte concentration is in the range of 12 to 20 g/l. Acidifying agent is added to the stock solution until pH value of between 7 and 8.5 is obtained. To this solution an acidifying agent and remaining silicate solution is simultaneously added in such a way that the pH of the reaction mixture does not change. Subsequently an additional amount of acidifying agent is added to the reaction mixture to bring down the pH of the reaction mixture in the range of 4.5 to 5.5. The temperature of the reaction medium is kept between 68° C. to 98° C. The drawback of the process is that initial addition of acid to bring down pH to below 8.5 is critical as it leads to probability of gel formation. Besides, in the simultaneous addition of solutions the rates are to be critically controlled to maintain the pH.
U.S. Pat. No. 6,180,076 (2001) assigned to Uhriahdt et al. describes a method for the preparation of readily dispersible precipitated silica having BET specific surface in the range of 120-200 m2/g, DBP index 150-300 and some of the particles are of less than 1 μm after degradation of the particles by ultra-sonication. The particles are readily dispersible having degradability ratio wK<3.4 μm. The wK coefficient defined as a ratio of peak height (maximum) of non-degradable particles in the range of 1 to 100 μm and a maximum of the degraded particles in the range of <1 μm after ultrasonic treatment at 100 W for 4.5 minutes. The precipitated silica is obtained by the reaction of alkali metal silicate with mineral acid at a temperature from 65° to 95° C. at pH of 7.0 to 11.0 with continuous stirring. The reaction is continued up to a solids concentration of 40-110 g/l and the final pH is adjusted to a value between 3 and 5. The product is filtered, washed and then dried. Reaction as per this invention is carried out in two steps: 1) Addition of water glass and acid for 15 to 25 minutes followed by interruption of the addition for 30 to 90 minutes. 2) Addition of water glass and acid for 50 to 70 minutes. The total reaction time is 130 to 140 minutes. The particle size obtained after ultra-sonication at 100 W for 4.5 min as per some of the embodiments is <5 μm. The time of reaction is comparatively longer and the particle size without ultra-sonication has not been mentioned.
U.S. Pat. No. 6,214,912 (2001) assigned to Chevallier, et al. describes a process for the preparation of silica including the reaction of a silicate of an alkali metal ‘M’ with an acidifying agent, whereby a suspension of precipitated silica is obtained, and then the separation and the drying of this suspension, the said process being characterized in that the precipitation is carried out in the following manner: (i) an initial stock is formed comprising a part of the total quantity of the silicate of an alkali metal ‘M’ involved in the reaction, the silicate concentration, expressed as SiO2, in the said stock being lower than 20 g/l, (ii) the acidifying agent is added to the said initial stock until at least 5% of the quantity of M2O present in the said initial stock is neutralized; (iii) acidifying agent is added to the reaction mixture simultaneously with the remaining quantity of alkali metal silicate such that the consolidation ratio, that is to say the ratio of quantity of silicate added (expressed as SiO2) per quantity of silicate present in the initial stock (expressed as SiO2) is between 12 and 100. In the stage (iii) the added quantity of acidifying agent is preferably such that 80 to 99% of the added quantity of M2O is neutralized. The reaction mixture is aged for 1 to 60 minutes. Finally, after the precipitation, an additional quantity of acidifying agent is added to the reaction mixture to reach the pH between 3 and 6.5. The drawback of this process is that during simultaneous addition of silicate and acid in stage (iii) it is difficult to adjust the quantity of acidifying agent that neutralizes 80 to 99% of the added quantity of Na2O, which makes the process complicated.
U.S. Pat. No. 5,851,502 (1998) assigned to Turk et al discloses that precipitated silica can be prepared by introducing water into a precipitation vessel, adding water glass until an alkali value is between 5-15 and simultaneously adding water glass solution and sulfuric acid until the pH of the reaction mass is around 8.5, interrupting precipitation for some time and then further continuing the neutralization with acid until the pH of the slurry is ca. 4. The drawback of the process is that both the solutions are to be added simultaneously and their rates are to be controlled very critically. Spray drying is required to obtain desired product, which required high-energy input.
Indian Patent No. 176707 (1996) assigned to Mody, et al. discloses a process for the preparation of precipitated silica at ambient temperature using hydrochloric acid. The process involves preparing aqueous solution of sodium silicate having 1 to 1.3 N Na+ ion concentration, adding 14-16% hydrochloric acid to the said solution at constant rate over a period of 10-60 minutes under continuous stirring to bring down the pH of the resultant mixture to around 10.8, continuing addition of the same acid for 3-6 hours to bring down the pH between 3 and 4 to obtain the precipitated silica. Finally, separating, washing, drying and pulverizing the said precipitated silica by known methods. The drawback of this process is that complete neutralization of alkali metal silicate solution at ambient temperature takes very long time, which makes process uneconomical. Besides, the process is technically feasible only with hydrochloric acid and not with other acids.
U.S. Pat. No. 5,342,598 (1994) assigned to Persello describes a process wherein the silica particulates are prepared by simultaneous addition of sodium silicate and a diluted acid into a dispersion of colloidal silica, under continuous agitation. At the completion of the reaction the pH of the suspension is between 3 and 7. In this process, colloidal silica is used as a nucleating agent and is separately prepared by heating a sodium silicate solution at around 75° C. and adding acid until the final pH is around 9.5. The drawback of the process is that both the solutions are to be added simultaneously and their rates are to be controlled very critically. Moreover, dilute colloidal silica is needed for precipitation of silica.
U.S. Pat. No. 5,094,829 (1992) assigned to Krivak et al. discloses a method for the preparation of reinforced precipitated silica having BET specific surface in the range of 220 to 340 m2/g with pore diameter of 9 to 20 nm. The process involves the neutralization of alkali metal silicate solution in multiple stages of precipitation and aging. About nine stages of reactions are involved where in certain stages the reaction mixture is brought to pH below 7 and again pH is raised to alkaline condition. Further precipitation by simultaneous addition of acid and silicate solution is commenced and again pH is brought to acidic condition. At least three aging steps are involved in the process. The reaction in first step is carried out at temperature below 50° C. and then raised to 80° to 95° C. The drawback of the process is that the reaction time is more than 160 minutes, which makes the process uneconomical. The particle size of the product is not mentioned.
U.S. Pat. No. 5,034,207 (1991) and U.S. Pat. No. 5,123,964 (1992) assigned to Kerner et al. describes a process for preparation of silica, having BET specific surface in the range of 150 to 350 m2/g, bulk density between 60 and 120 g/liter and at least 70% particles having size of 1 to 6 μm, can be obtained by heating water glass solution at 700 to 80° C. under agitation, adding concentrated sulfuric acid into water glass solution until half of the alkali present has been neutralized, shearing the reaction mixture and optionally raising the temperature at the same time to 86° C. Addition of acid is interrupted for 30 to 120 minutes and acid addition is continued until the pH of suspension is 3 to 3.5. The silica suspension is optionally diluted with water and a centrifugal pump and a hydrocyclone separates the coarse particles. The silica is separated by filtration, washed and again suspended in water and spray dried. The suspension during the reaction is sheared by passing it through shearing unit (e.g. Dispax reactor) and reintroduced from the top of the reaction vessel. In this prior art the suspension is sheared through shearing unit, which requires high energy input and that also add to the capital cost.
U.S. Pat. No. 4,495,167 (1985) assigned to Nauroth et al. describes a process for the preparation of precipitated silica having specific surface area higher than 400 m2/g, DBP number higher than 300 and more than 99% particles <63 μm. The products are prepared by addition of sodium silicate and sulfuric acid simultaneously in such a way that the water present in the reaction vessel has pH in the range of 6-7 at 40° to 42° C., while stirring. The reaction suspension is sheared with a turbine type shearing device during the whole course of reaction i.e. 146 minutes. After 13th minute, addition of acid and silicate solution is interrupted for 90 minutes and again resumed at the same rate, which was previously used to obtain final silica concentration of 46 g/liter. The suspension of the product is then aged at room temperature for 12-17 hours. The product is separated by filtration, washed and re-suspended in water and spray dried. The product obtained by jet milling showed almost 99.99% particles having particle size less than 63 μm. However, particle size distribution of the particles less than 63 μm size has not been mentioned in the invention. This process takes 12 to 19 hours long time and use of shearing requires high energy input and additional capital cost for the process.
U.S. Pat. No. 4,243,428 (1981) assigned to Donnet et al. describes a process to obtained silica by neutralization of a solution of sodium silicate with an acid under conditions which influence the properties of the silica finally obtained. The products obtained having a specific surface area ranging between 100 and 600 m2/g, which do not vary upon drying, due to the action of a strong acid on an alkaline silicate, characterized by the fact that the addition of strong acid to the alkaline silicate is interrupted one to three times. According to another embodiment of this prior art the temperature of the reacting medium is varied during two phases in which strong acid is successively added to the silicate, separated by one interruption. Another embodiment of this prior art is the speed at which the acid is added. According to a preferred embodiment of this prior invention to obtain silica having particle size less than about 20 μm, the acid is added to the silicate at a rate varying according to a law such that the residual alkalinity and the concentration of the total silica decrease linearly as a function of the reaction time, in accordance with the following equation in which:
  D  =                    Vi        ⁡                  (                                    Ci              ⁢                              /                            ⁢              62                        +                          Ca              ⁢                              /                            ⁢              98                                )                    ⁢      Ci      ⁢              /            ⁢      62                      T        ⁡                  (                                    Ci              ⁢                              /                            ⁢              62                        +                          Ca              ⁢                              /                            ⁢              98                        -                          Ci              ×              t              ⁢                              /                            ⁢              62              ×              T                                )                    2      D=flow at any time ‘t’; Vi=volume of initial silicate solution; Ci=Na2O concentration of the initial silicate expressed in g/liter; Ca=concentration of the acid used, expressed in g/liter; T=total time of the reaction; t=time in minutes. However, in order to maintain the said condition the rate of addition of acid is continuously changed during the course of reaction and hence the process becomes complicated.